Since you find them so interesting…there’s a “young adult” series of three books called “The Secret Circle” by L. J. Smith, in which a crystal skull plays a very major part in the plot. The story itself is about a clique of high school witches, and quite well written – I found it delicious to read and I am well beyond the targeted age demographic! One of the things I really loved about it was the correspondences…each girl in the group is gradually revealed to have a color, gemstone, and (classical) Goddess association!!! The magical details in the series are given so naturally and believably, yet abundantly, and really help create a complete world unto itself, that pulls the reader in.

L.J. Smith has also written a number of other paranormal-themed series; her “Vampire Diaries” one has become quite well known!